1. Technical Field
The invention relates to semiconductor devices and in particular to devices involving InP.
2. Art Background
Semiconductor laser devices based on InP materials, i.e., materials including both indium and phosphorus, such as InP, indium gallium phosphide and indium gallium arsenide phosphide, are of particular significance in optical communication applications. Such lasers are generally formed by depositing a series of layers including a n-type InP layer, a quaternary alloy region forming the layer, a p-type InP layer and generally a quaternary indium gallium arsenide phosphide overlying layer. These layers are then patterned to form a mesa such as shown in FIG. 1 with the active region denoted as 1, the n and p InP layers denoted 2 and 3, respectively, and the quaternary overlying layer denoted 4. The sides of the mesa are passivated to prevent contamination, to ensure mechanical integrity for subsequent processing, and provide the refractive index difference required for efficient waveguiding in the active layer. However, the materials and structure used are also chosen to ensure the excessive current leakage, i.e., a current greater than 10 .mu.A, does not flow from the mesa region through the passivation area to the substrate or to another mesa region.
Generally, this isolation is performed by forming in the isolation region, 7 in FIG. 2, a series of reverse-biased p-n junctions and/or semi-insulating regions from appropriately doped InP materials. If such InP materials are directly deposited onto the substrate containing the mesas, unacceptable growth structures such as shown in FIG. 3 are obtained. Such structures are produced since the deposited InP not only grows in the region 7 but also grows on the mesa 8. To avoid such structures, a mask, 9 in FIG. 4, that substantially overhangs the mesa by generally dimensions of approximately 2 .mu.m, is formed before passivation region growth by undercutting an etching mask layer during mesa formation. The overhang prevents the growth structure shown in FIG. 3 obtained where a mask (not shown) is present over a mesa of the depicted geometry. However, to produce the undercut region, wet etching rather than etching in a plasma is required. Etching in a plasma is preferred since it is generally more easily controlled and generally leads to improved morphology in adjacent regions. Although a selective growth process which allows growth only in passivation areas and not on the mesas would eliminate the need for the overhang structure shown in FIG. 4, selective growth is generally not available for InP, and thus etching in a plasma is precluded.
As discussed, selective growth of InP is desirable for laser production and is also considered useful for applications such as opto-electronic integrated circuit fabrication. Various approaches have been employed for selective growth of GaAs but these approaches are not analogously useful for InP. For example, GaAs in metalorganic chemical vapor deposition (MOCVD) is formed utilizing precursors such as arsine and triethyl gallium. As described by Kuech et al, Journal of Crystal Growth, 99, 324 (1990), the use of the corresponding chloride, e.g., diethyl gallium chloride for the source of gallium allows selective growth of GaAs relative to silicon nitride or silicon dioxide.
The analogous reaction to form InP, however, has severe limitations. Diethyl indium chloride has a vapor pressure of less than 1 torr at 50.degree. C. Therefore, to supply sufficient indium precursor, i.e., to supply a flow greater than 0.1 sccm, the diethyl indium chloride must be heated to a temperature substantially above room temperature. To prevent condensation of the resulting flow in the deposition equipment, the entire gas introduction path must be correspondingly heated. This requirement although feasible is generally not practical for commercial applications.
Research into selective deposition of GaAs has also involved the use of chlorine in the arsenic precursor. For example, as described by Azoulay and Dugrand, Applied Physics Letters, 58 (2), 128 (1991), arsenic trichloride was employed with trimethyl gallium and AsH.sub.3 to produce a selective growth of GaAs on a semiconductor region relative to a masked region. Analogous use of phosphorus trichloride for selective growth of InP is again inconvenient. The phosphorus trichloride is relatively corrosive and requires substantial complication in the construction and operation of the deposition equipment.
Therefore, although a selective approach for the deposition of InP is quite desirable, a suitable method has not been reported.